Sydney Morning Herald political and international editor

Joe Hockey is escalating the pressure on the Gillard government to revisit its troubled mining tax, threatening to change the law to force disclosure of the revenue it is raising.

In an unorthodox move, the Senate this week ordered the Tax Commissioner, Chris Jordan, to disclose the yield from the tax by the end of next week.

The government budgeted that the tax would raise $2 billion in the current financial year, but mining company executives have remarked privately that none of the major miners is caught by the tax.

Industry speculation is that the tax is yielding a negligible sum. Mr Hockey, the shadow treasurer, likes to say that ''only Wayne Swan could create a new tax that doesn't raise any revenue and has brought down two prime ministers''. The Australian Tax Office, which has refused to disclose the information on privacy grounds, said on Thursday that it was taking legal advice.

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But now Mr Hockey has lodged a Private Member's Bill with the House of Representatives to increase the pressure on the commissioner and the government.

The bill, if passed, would amend the law to force the Tax Office to disclose the total sums raised by the mining tax, quarter by quarter.

And it would oblige the government, in turn, to publish the information.

Although Labor would oppose the bill, it would be supported by the Greens MP Adam Bandt. Two independents, Rob Oakeshott and Tony Windsor, are in favour of the disclosure in principle, so the bill would stand a good chance of passing. The Greens and the Coalition would then combine in the Senate to vote it into law.

The parties have different purposes, however. The opposition hopes that exposing the value of tax raised will discredit the tax. Mr Hockey said: ''We will abolish it. It's a failed tax.'' But the Greens, who hold the balance of power in the Senate, want to add heft to the tax. The Greens hope that exposing the tax as a failure will create pressure on the government to redesign it to raise more revenue.

Mr Oakeshott said on Thursday that he would like to know the value of the tax raised, but would reserve judgment until he could discover whether any shortfall was because of moves in commodity prices or because of structural flaws in the tax.

If there were a structural problem, he would support a revision of the tax, but said: ''In the nine weeks of Parliament left before the election, I don't think either of the two main parties will have the bottle to deal with it.''

The chairman of Australia's third biggest iron ore miner, Andrew Forrest, has said that the crucial weakness of the tax is that it allows mining firms to deduct the value of their ore reserves from their tax liability.

The big miners have vast ore reserves valued in the billions and might not have to pay the tax for many years, he said.

Another flaw of the tax's design is that if a state government increases its own tax on mining companies through royalty charges, the federal government is obliged to compensate the miners.

The Tax Office and the Treasury have refused to disclose the amount of revenue harvested by the tax, citing legal requirements to protect taxpayer secrecy.